Unit fuel injectors operated by cams, have long been used in compression ignition internal combustion engines for their accuracy and reliability. The unit injector typically includes an injector body having a nozzle at one end and a cam driven injector plunger mounted for reciprocating movement within the injector body. In the typical unit fuel injector, a link, which is cam actuated, physically communicates with a lower, intermediate or upper plunger which moves inwardly, during the injection event, to force fuel either into an injection chamber and out an injector orifice or directly out of an injector orifice on a cycle-by-cycle basis. To achieve optimal engine operation fuel must be injected at very high pressure to cause the maximum possible atomization of the injected fuel. In addition, the interval of injection needs to be carefully timed during each cycle of injector operation in dependence upon the movement of the corresponding engine piston.
Internal combustion engines are subjected to a variety of external as well as internal variable conditions ultimately affecting the performance of the engine. Examples of such conditions are engine load, ambient air pressure and temperature, timing, power output and the type and amount of fuel being consumed. In order to satisfy the increased need for higher engine efficiency and pollution abatement, accurate control over and a means for varying (1) the timing of injection, (2) the metering of the proper quantity of fuel and (3) the injection pressure in response to changing engine operating conditions is required.
Attempts to provide independent control over these parameters from one cycle to the next have, in most cases, been unsuccessful due, in part, to the way in which fuel is supplied to the injector. In most cases fuel is pumped from a source by way of a low pressure rotary pump or gear pump to the unit injector which may be thought of as a high pressure pump. Such high pressure pumps conventionally include a positive displacement piston driven by a cam which is mounted on an engine driven cam shaft. High pressure pumps of the electrical, mechanical, hydraulic or electromechanical types are known as well, however, these systems often lack reliable independent control over the various injection parameters from cycle to cycle.
Other attempts to independently vary these key injection parameters have, in many cases, failed due to their dependence upon other engine operating conditions. For example, injection pressure, in the typical unit fuel injector, is dependent upon the velocity of the inward movement of a cam actuated injector plunger during the injection event. In unit fuel injectors of this type, the injector plunger is mechanically connected to the engine cam shaft and, as a result, injection pressure is dependent upon engine speed. Therefore, the injection pressure cannot be adequately varied for each cycle of injection operation to provide improved efficiency in engine operation and pollution abatement.
A well known approach to solving the lack of cycle-by-cycle control capability is to employ a solenoid valve in combination with the unit injector to vary the quantity and timing of injection during each cycle. For example, in U.S. Pat. Nos. 4,129,253 to Bader et al. and 4,392,612 to Deckerd et al., an electromagnetic unit fuel injector is disclosed including a single, cam operated injector plunger, an electromagnetic valve for determining the beginning and ending of injection, and thus, the timing and quantity of fuel injected during each cycle of plunger movement, and a tip-mounted valve for resisting blow back of exhaust gases into the high pressure chamber of the injector while allowing fuel to be injected into the cylinder. Injector assemblies of this type are often referred to as jerk-type unit injectors.
As is shown in the above-mentioned patents, injection pressure is controlled and determined by a fixed displacement pump structure so as to permit the intensification of the fuel pressure and injection of fuel to provide both a pilot and a main charge injection. Although the fuel pressure levels obtained during both high load and low load engine operation is sufficient to provide for injection, the fixed displacement and volume of fuel supplied by the unit injector pump does not allow for high accuracy in the control of the timing of injection or metering of a quantity of fuel under varying conditions at or close to maximum peak pressures. The inability of these types of injectors to operate at maximum peak pressures under varying conditions, from low load to high load engine operation, results in a degradation of the engines ultimate performance.
Other unit fuel injection systems attempt to solve the lack of cycle-by-cycle control capability by varying the quantity and timing of injection during each cycle by a collapsible hydraulic link to selectively change the effective length of the cam operated fuel injector plunger. For example, in U.S. Pat. No. 4,463,901 to Perr et al., a unit fuel injector is disclosed including a three part, cam operated injector plunger defining within an internal bore a variable volume injection chamber, a variable volume timing chamber and a variable volume compensation chamber in which is mounted a biasing means for biasing the plunger sections defining the compensation chamber in opposite directions to collapse the timing and injection chambers. Control and variation of the timing of injection for each cycle of injection operation is achieved in dependence upon the volume of fuel supplied to the timing chamber, thereby defining the length of the hydraulic link formed therein. The amount of fuel is independently controlled by the volume of fuel supplied to the injection chamber. The amount of fuel supplied to the respective timing and injection chambers is affected by the spring constant of the biasing spring located in the compensation chamber. While providing for accurate independent control and variation of the timing of injection and metering of the proper quantity of fuel, the unit fuel injector of Perr et al. '901 does not allow for variation of injection pressure for each cycle of injection operation in response to the changing engine operating conditions, independent of engine speed. Similar types of unit fuel injectors including two-part plunger assemblies are disclosed in U.S. Pat. No. 4,531,672 to Smith, U.S. Pat. No. 4,281,792 to Sisson et al. and U.S. Pat. No. 4,235,374 to Walter et al.
In the typical unit fuel injectors, such as those discussed above, the actual cycle-by-cycle injection of the pressurized fuel through the injector orifice is achieved by inward movement of a plunger connected to a link driven by the engine cam shaft during the injection event. Injection pressure for each cycle of injection operation for injectors operating in this manner is dependent upon engine speed. Control over and variation of this parameter is necessary to achieve optimal engine operation and is not possible where such control and variation is dependent on engine speed. In addition, to achieve and maintain the maximum peak pressure to ensure maximum possible atomization of the injected fuel, the plunger, which travels inwardly during the injection event, must travel inwardly with an extremely high velocity and the injection event must occur over a relatively short time span. The typical time interval for the injection event is in the range of 2-4 milliseconds. High velocity movement of the plunger in a short time period requires a high rate of acceleration, which, in a cam actuated unit fuel injector, is determined by the cam profile.
As is well known, the contour or shape of the lift ramp of the cam Will determine the rate of acceleration of the plunger. To achieve the necessary high velocity in a short period of time, a high rate of acceleration is required which can only be achieved by a cam lobe exhibiting very sharp radii of curvature (i.e. sharply angled lift ramp). The lift profile of the cam in a fuel injector that injects fuel in this manner is characterized by a lift ramp having a very sharp angle which is disadvantageous in that such a design greatly increases cam hertz stresses, resulting in increased wear on the cam and cam follower surfaces.
Attempts have been made to provide a unit fuel injector in which the injection event does not occur concurrently with the inward movement of a plunger connected to the engine cam shaft (i.e., the cam, link and plunger assemblies), thereby, eliminating the need for the higher rates of acceleration required by the fuel injectors described above. For example, U.S. Pat. No. 4,275,693 to Leckie discloses a fuel injection timing and control device wherein injection of fuel, which is pressurized by way of a plunger/piston arrangement, is carried out by the use of a solenoid controlled sleeve tip valve, wherein the solenoid is mounted coaxially with the central axis of the injector body. Fuel is supplied to an accumulator, which is provided with a piston slidably disposed in a bore, movable upwardly against the bias of a spring and a relief valve to relieve pressure within the accumulator above a predetermined level. The fuel is continuously maintained at a constant pressure level during the preinjection, injection and post-injection events. When the solenoid is activated, the tip valve allows a metered portion of the pressurized fuel in the accumulator to be injected through discharge passages. Pressurization of fuel within the accumulator of the injector is disassociated from timing and duration of fuel injection, which is controlled solely by the energization of a solenoid. Injection, thus, occurs independently of any mechanical connection to the cam shaft.
In the operation of the fuel injector disclosed in Leckie, the pressure of the fuel in the accumulator is relatively constant, resulting in a corresponding relatively constant injection pressure. Injection pressure is controlled by the spring constant of the spring biasing the piston defining the accumulator in conjunction with the relief valve arrangement. Therefore, while independent of engine speed, the fuel injector of Leckie cannot allow for variation of injection pressure for each cycle of injection operation in response to engine operating conditions to optimize engine efficiency and pollution abatement. Moreover, Leckie's use of a pressure relief valve causes the excess energy stored in the fuel within the accumulator to be wastefully lost upon opening of the relief valve.
Consequently, there is a need for a unit fuel injector Wherein the injection event does not necessarily occur concurrently with the inward movement of a plunger connected to the engine cam shaft, in which the accurate control over and variation of the timing of injection, the metering of the proper quantity of fuel and the injection pressure is possible independent of engine speed.